CH. xxiv.] RESPIRATORY NERVES. 359 



the centre by venous blood. It is not due (or not wholly due) 

 to the action of the venous blood on the terminations of the vagi 

 in the lungs, as the same phenomenon occurs when these nerves 

 are cut ; and, moreover, dyspnoea takes place if the venous blood is 

 allowed to circulate through the brain alone, and not through the 

 lungs at all. For instance, it ensues when localised venosity of 

 the blood is produced in the brain by ligature of the carotid and 

 vertebral arteries. 



But, as before stated, the normal activity of the respiratory 

 centre is not automatic, it is reflex, and the principal afferent 

 channel is the vagus. The way in which it works has been made 

 out of recent years by Marckwald, Hering and Head. The 

 following is a brief resume of Head's results. 



His method of recording the movements was by means of that 

 convenient slip of the diaphragm which is found in rabbits (see 



P- 353)- 



His method of dividing the vagus was by freezing it ; he laid 

 it across a copper wire, the end of which was placed in a freezing 

 mixture. This method is free from the disadvantage which a cut 

 with a knife or scissors possesses, namely, a stimulation at the 

 moment of section. On dividing one vagus, respiration became 

 slightly slower and deeper ; on dividing the second nerve, this 

 effect was much more marked. 



On exciting the central end of the divided nerve, inspiratory 

 efforts increased until at last the diaphragm came to a standstill 

 in the inspiratory position. But if a weak stimulus was employed, 

 the reverse was the case ; the expiratory efforts increased, inspira- 

 tion becoming weaker and weaker, until at last the diaphragm 

 stopped in the position of expiration. This result always follows 

 stimulation of the superior laryngeal nerve. 



Most of these facts were known previously, but the interpreta- 

 tion of them, in the light of further experiments immediately to 

 be described, is the following : 



That there are in the vagus two sets of fibres, one of which pro- 

 duces an increased activity of the inspiratory part of the respira- 

 tory centre, and the other an increased activity of the expiratory 

 part of that centre. Stimulation of the first stops expiration and 

 produces inspiration ; stimulation of the second does the reverse. 



The question now is, What is it that normally produces this 

 alternate stimulation of the two sets of fibres ? If we discover this 

 we shall discover the prime moving cause in the alternation of the 

 inspiratory and expiratory acts. It was sought and found in the 

 alternate distension and contraction of the air-vesicles of the lungs 

 where the vagus terminations are situated. 



